The present invention relates to an optical scanning device to be used in a laser beam printer and the like.
In an laser beam printer, an optical scanning device emits a laser beam that is modulated with print information. More specifically, in the laser beam printer, a laser diode is switched ON and OFF in accordance with data representing the print information. The laser beam emitted by tile laser diode is deflected by a polygonal mirror towards a circumferential surface of a photoconductive drum so as to scan it.
In the optical scanning device, an unmodulated laser beam is emitted and then detected by a detection means to establish a starting reference for the scanning of the photoconductive drum by a modulated laser beam. Thus, a light receiving element of the detection means, such as a photo sensor, is provided on the upstream side of the photoconductive drum with respect to the scanning direction. The light receiving element receives the unmodulated laser beam and adjusts the starting reference used for the emission of the modulated laser beam. The emission of the modulated laser beam is then started a predetermined period after the reception of the laser beam by the light receiving element of the detection means.
The detection means outputs a detection signal when it receives the laser beam. In a conventional optical scanning device, the emission of the unmodulated laser beam is terminated after the output of the detection signal is finished.
Also, in a conventional optical scanning device, there is a delay between the reception of the laser beam by the detecting means and the output of the detection signal as a result of the response characteristics of the circuitry of the light detecting device.
Thus, in the conventional optical scanning device, the laser beam received by the detection means and used to establish the starting reference for the modulated laser beam, keeps emitting light even after it has passed the light receiving area of the detection means.
If such an optical scanning device as employed in a high-speed scanning type laser beam printer, an imaging area of the circumferential surface of the photoconductive drum may be scanned which should not have been scanned. If the photoconductive drum is exposed to the laser beam as above, a latent image is formed, and developed in the developing process. Accordingly, for example, a belt-like image would be printed on a recording sheet.
If an out-of-imaging area of the photoconductive drum is scanned, the additional image, such as the belt-like image, would not be printed on the recording Sheet. However, since the latent image is formed on the out-of-imaging area, and therefore a toner image is formed on the photoconductive drum, toner is wasted, the efficiency of the cleaning function for the circumferential surface of the photoconductive drum is reduced, and the possibility of dirt inside the printer is increased.